Revert "Use child_process.exec() instead of spawn() in FileFinder"

child_process.exec kills the subprocess after 200 kB of stdout, which is not a
high enough threshold in many cases:
http://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback.

This reverts the lib/FileFinder.js changes in commit
40b39985efa7ffb084207dc1a8c1b86bd2c0725c and bumps the package version number.